Maneera,
I don't get to write on the forum often...but HELLO! Congrats on your exciting trip!
To the marvellous advice of everyone, might I add:
UK: Would a simple trench rain coat (preferably with hood) suffice with your summer wear? I recall reading a blogger a few years back who used her well-fitting, color-flattering trench as her fashion statement. I'm an American in my late 50s, and years ago when we traveled abroad--particularly to the UK (before today's big layering trend)-- we simply wore a COAT (haha, what a concept)! Also, I second (or third, fourth, and fifth) the careful shoes and scarves advice for back and foot health, warmth and color!
Thailand: I have only been to Bangkok, but I went in August--very similar to your time frame. As you know, because of humidity, you will need the gauziest natural fabrics possible. They don't call the Southeast Asian climate a jungle for nothing! However--and here's my best tip--be sure to cover your shoulders, knees and FEET in public places--NO SANDALS in public establishments. Travelers were refused entry to restaurants for such "inappropriate" dress in a Buddhist country. That said, at Temples, you are required to remove footwear before entering (but as a woman must always cover shoulders and knees). Other travel tips: Always have tissues on you as emergency toilet paper; don't accept bottled water with caps already open (they are recycled); beware of tuk tuk drivers who offer to cheaply show (and then hijack) you the city at a bargain. They make a deal with specific shop owners to bring tourists to their shops. They give you code words to say (tell them you are here for X) so the shop owners can give the appropriate agent a kick-back. We got hornswaggled into one of these and had to pay off the driver to let us go. They'll take you to parts unknown and then you are dependent on them to get you back! That said, the food is amazing, as are most of the people, especially if you are in trouble. Health care is excellent and out-of-pocket cheap (a member of our party had a short hospital stay). Just know Bangkok is the city of illusions, from knockoff luxury goods to feints, such as the fraud described above! Be safe, be careful, have fun!